Hydrogenation of Alkyl Carboxylic Acids with Tetrahydropyrimidine-Derived Iridium Complexes under Mild Conditions

The hydrogenation of carboxylic acids to alcohols is of great significance in synthetic chemistry. Reported herein are competent iridium catalysts [Cp*Ir(N,N ')(H2O)][OTf](2) (N,N ' = three different 2,2 '-bi-1,4,5,6-tetrahydropyrimidines) for application in the catalytic hydrogenation of a series of alkyl carboxylic acids under mild conditions (25-100 degrees C) without any additives. Among these structurally diverse catalysts evaluated for reactivity, complex 4 bearing an electron-deficient ligand (N,N ' = 2,2 '-bi-5,5 '-hydroxyl-1,4,5,6-tetrahydropyrimidine) is the most robust, which can hydrogenate acetic acid smoothly at 25 degrees C to give the alcohol and ester products with a TON of 1106. Detailed mechanistic studies suggest that the generation of an [Ir-H] intermediate is involved in the rate-limiting step. With this catalyst system, a broad range of alkyl carboxylic acids were hydrogenated with moderate to high TONs.

Automated summary

This article reports an efficient iridium catalyst for the hydrogenation of alkyl carboxylic acids to alcohols and esters under mild conditions without any additives. Among the evaluated catalysts, complex 4 shows the highest performance with a TON of 1106 for acetic acid hydrogenation.